How to do multiple shells in my Prism app (like MS Office)?

I try to create an application that has a window behaviour as MS Office, for example Word/Excel. The user opens the application and when clicking new, a completely new window shall appear with the look of the application.

The closest that I found so far is this: Link

However, here the shells are shown at application start. How to do this by command, or maybe there is a completely different way to achieve this?

Edit: I have also found the following now: Link, but where and how to call this code?